The subject matter of the Patent Application in accordance with WO 02/43224 A1 with the priority date Nov. 21, 2000 and entitled “Supraleitungseinrichtung mit einem thermisch an eine rotierende, supraleitende Wicklung angekoppelten Kaltkopf einer Kälteeinheit” [Superconducting device having a cold head, which is thermally coupled to a rotating, superconducting winding, of a refrigeration unit], which was not published prior to this, is a special superconducting device with a rotor which is mounted such that it can rotate about a rotation axis and has at least one superconducting winding with conductors (which are arranged in a thermally conductive winding mount) as well as a refrigeration unit, in which at least one cold head is thermally coupled via thermally conductive parts to the winding, making use of a thermosyphon effect.
In addition to metallic superconductor materials such as NbTi or Nb3Sn which have been known for a long time and have very low critical temperatures Tc and which are therefore also referred to as low Tc superconductor materials or LTS materials, metal-oxide superconductor materials with critical temperatures above 77 K have been known since 1987. The latter materials are referred to as high-Tc superconductor materials, or HTS materials, and in principle allow a refrigeration technique using liquid nitrogen (LN2).
Attempts have also been made to create superconducting windings with conductors using such HTS materials. However, it has been found that already known conductors can carry only a comparatively small amount of current in magnetic fields with inductions in the Tesla range. This often means that it is necessary to keep the conductors of windings such as these at a temperature level below 77 K, for example between 10 and 50 K, despite the intrinsically high critical temperatures of the materials being used in order in this way to make it possible to carry significant currents at field strengths of several Tesla. A temperature level such as this is admittedly on the one hand considerably higher than 4.2 K, the boiling temperature of liquid helium (LHe) with which known metallic superconductor elements such as Nb3Sn are cooled. On the other hand, however, cooling with LN2 is uneconomic due to the high conductor losses. Other liquefied gases such as hydrogen with a boiling temperature of 20.4 K or neon with a boiling temperature of 27.1 K cannot be used owing to their danger or their lack of availability.
Refrigeration units in the form of cryogenic coolers with closed helium compressed gas circuits are therefore preferably used for cooling windings with HTS conductors in the temperature range. Cryogenic coolers such as these are, in particular, in the form of the Gifford-McMahon or Stirling type, or are in the form of so-called pulse tube coolers. Refrigeration units such as these also have the advantage that the refrigeration performance is available virtually at the push of a button, and there is no need for the user to handle cryogenic liquids. When refrigeration units such as these are used, a superconducting device such as a magnet coil or a transformer winding is cooled only indirectly by heat conduction to a cold head of a refrigerator (see, for example, “Proc. 16th Int. Cryog. Engng. Conf. (ICEC 16)”, Kitakyushu, JP, May 20-24 1996, Verlag Elsevier Science, 1997, pages 1109 to 1129).
A corresponding cooling technique is also provided for a superconducting rotor of an electrical machine which is disclosed in U.S. Pat. No. 5,482,919 A. The rotor contains a rotating winding composed of HTS conductors, which can be kept at a desired operating temperature of between 30 and 40 K by a refrigeration unit which is in the form of a Stirling, Gifford-McMahon or pulse tube cooler. In a specific embodiment for this purpose, the refrigeration unit contains a cold head which also rotates, is not described in any more detail in the document, and whose colder side is thermally coupled to the winding indirectly, via elements which conduct heat. Furthermore, the refrigeration unit of the known machine contains a compressor unit which is located outside its rotor and supplies the cold head with the necessary working gas via a rotating coupling, which is not described in any more detail, of a corresponding transfer unit. The coupling also supplies the necessary electrical power via two sliprings to a valve drive, which is integrated in the cold head, of the refrigeration unit. This concept makes it necessary for at least two gas connections to be routed coaxially in the transfer unit and means that it is necessary to provide at least two electrical sliprings. Furthermore, the accessibility to the rotating parts of the refrigeration unit and, in particular, to the valve drive in the rotor of the machine is impeded, since the rotor housing must be open when servicing is necessary. Furthermore, the operation of a known valve drive with fast rotation, as in the case of synchronous motors or generators, is not assured.
In order to ensure reliable and economic operation of a refrigeration unit both when the rotor is at rest and when it is rotating, in a temperature range below 77 K and with less hardware complexity, the subsequently published patent application according to WO 02/43224 A1 proposes the following features for the superconducting device of the type mentioned initially:                the winding mount should be equipped with a central, cylindrical cavity which extends in the axial direction and to which a lateral cavity is connected which leads out of the winding mount,        the cold head should be located in a fixed manner outside the rotor and thermally connected to a condenser unit for condensation of a refrigerant,        a stationary heat pipe should be coupled to the condenser unit, which pipe projects axially into the corotating lateral cavity and seals off this area radially, and        the heat pipe, the lateral cavity and the central cavity should be filled with coolant, with condensed refrigerant being passed, when making use of the thermosyphon effect, via the heat pipe into the lateral cavity and from there into the central cavity, and refrigerant which is vaporized there being passed back via the lateral cavity and the heat pipe to the condenser unit.        
In consequence, in this refinement of the proposed superconducting device, the entire refrigeration unit is arranged with any moving parts outside the rotor, and is thus easily accessible at any time. The refrigeration power and the heat transfer are provided by a stationary cold head in the rotor via the heat pipe, which ensures that the refrigerant is transported without any mechanically moving parts. In this case, the refrigerant is condensed, with heat being emitted, in a circulating process in a condenser unit, which is connected in a highly thermally conductive manner to the cold head. The liquid condensate then runs through the heat pipe into the lateral cavity and from there into the central cavity in the rotor. The condensate is transported through the heat pipe under the influence of the force of gravity on the basis of a so-called thermosyphon effect, and/or by the capillary force of the inner wall of the heat pipe. In this context, this pipe acts in a manner which is known per se as a “wick”. This function can also be optimized by appropriate refinement or cladding of the inner wall. The condensate drips into the lateral cavity at the end of the heat pipe. This condensate, which is passed from this lateral cavity into the central cavity, which is located in the region of the winding, is at least partially vaporized there. The refrigerant, which is vaporized in this way with heat being absorbed, then flows through the interior of the heat pipe back into the condenser device. The return flow is in this case driven by a slight overpressure in the central cavity, which acts as an evaporator part, relative to the parts of the condenser unit which act as a condenser. This reduced pressure, which is produced by the creation of gas in the evaporator and by the liquefaction in the condenser, leads to the desired refrigerant return flow. Corresponding refrigerant flows are known from so-called heat pipes.
The advantages of this refinement are, inter alia, that there is no need for any moving parts, such as fans or pumps, to circulate the refrigerant. Furthermore, only a single thermally insulating heat (transport) pipe, which can be designed to be relatively thin, is required to circulate the refrigerant. This reduces the complexity, particularly of the rotating seal, which seals the gas area of the refrigerant from the external area of the rotor. A seal, which is thus only comparatively small, is more reliable and requires less servicing since its circumferential speed is lower. In this case, gas losses of the refrigerant from the internal area to the external area have no significant influence on the operation of the heat pipe, since the amount of liquid in the system is effected only to a minor extent. In consequence, long lives can be achieved with an adequate reservoir size. Furthermore, the refrigeration unit can easily be matched to the different requirements of machine installation. In particular, depending on the configuration, a heat pipe with a length of many meters can be provided so that, for example, a refrigeration machine can be installed at an accessible point in order to simplify its servicing, while the actual motor or generator is installed in a location where access is difficult. The heat transfer and the provision of the refrigeration power are thus particularly simple and economic with the refinement, in particular since only a comparatively simple seal is required.
When cooling a superconducting rotor down from room temperature to the operating temperature by a cold head which is connected via only one thermosyphon, as is envisaged in the proposed superconducting device, the power of the cold head is relatively low at the low operating temperature of the thermosyphon, which is typically about 30 K. This results in correspondingly long cooling-down times. This is because, in a cooling system with only one thermosyphon, it is either necessary to tolerate a cooling-down time on the scale of roughly one or more weeks—depending on the cold mass and the refrigeration power—or the thermosyphon must be temporarily filled with a different working gas for initial cooling of the rotor, thus allowing a higher operating temperature. The latter procedure would necessitate several hours of maintenance work during a cooling-down process. However, this should be avoided, for logistics and cost reasons.